Conventional processes employed for removing microbial particles such as yeast or bacterial cells from aqueous solutions include gel filtration processes, centrifugal separation processes, adsorptive separation processes, precipitation processes and membrane filtration processes. However, gel filtration processes are difficult to apply for industrial purposes because the target substances are diluted by the solvents used for gel filtration, rendering such processes unsuitable for bulk treatment. Centrifugal separation processes can only be applied for microbial particles with sizes of several μm or greater, and for aqueous solutions of low viscosity. Adsorptive separation processes can be utilized for removal of certain trace microbial particles, but the processes cannot be applied for aqueous solutions in which many different microorganisms are dispersed in large quantities. Precipitation processes can be utilized for treatment of relatively large amounts of aqueous solutions, but microbial particles cannot be completely removed by such processes alone.
On the other hand, membrane filtration processes that employ microfiltration membranes or ultrafiltration membranes can remove various types of microorganisms and are suitable for industrial use as they permit continuous bulk treatment.
Examples of membranes used in such membrane filtration processes include polysulfone hollow fiber membranes having large pore sizes on the inner surface and having small pore sizes on the outer surface, as described in Patent document 1 or Patent document 2.